Dynamic steering and focusing of ultrasound arrays is essential in imaging and therapeutics to maintain resolution and pressure gain across an imaged or treated volume. As the array aperture increases in size to achieve the high lateral resolution needed for focusing at a distance, so does the number of elements required to form a tight beam with low side lobes.
Various fixed acoustic gratings have been fabricated by etching or shaping an acoustic material. These fixed gratings use a single frequency transmitter impinging on the grating structure to produce a particular steering angle or focusing of sound. U.S. Pat. No. 7,385,711 describes a blazed grating made out of solid material that is used to characterize liquid properties by measuring the attenuation and angle of the ultrasound beam that reflects or transmits through the grating. A number of physical zone plates and Fresnel phase plates have also been designed for ink drop ejection (U.S. Pat. No. 5,041,849) and ultrasonic therapeutics (U.S. Pat. No. 5,817,036). While the approach of using a machined material does reduce the complexity of the electronics, it does not allow for variability of the ultrasound beam.
U.S. Pat. Nos. 4,011,747 and 4,329,876 describe a set of fixed grating with a chirped surface acoustic wave or chirped bulk acoustic wave to produce a focused bulk acoustic wave that scans in the linear direction at the speed of sound of the surface wave or bulk wave, respectively. While this grating produces a scanned focus, the rate of scan is not variable, since it is dependent on material properties. Also, the focus can only scan effectively in one direction across the linear grating. This lack of flexibility prohibits the dynamic electronic control needed to scan a whole volume image.
Other approaches to manufacturing gratings, zone plates, and phase plates have involved modification of the transducer. U.S. Pat. No. 4,129,799 describes an approach to pole neighboring Fresnel zones of the transducer to be 180 degrees out of phase. Then actuation of the whole plate by a singular voltage source will produce a focused point of sound. While the electronics is simplified by only having one transmitting/driving signal, once the transducer is poles in a particular configuration, it cannot be modified dynamically to perform a different focusing or steering feature.
U.S. Pat. Nos. 3,911,730; 4,307,613; and 5,540,230 describe another way of producing a grating by modifying the transducer is to separate the electrodes or dicing elements in different regions of the transducer area. Applying signals with different amplitudes and phases to neighboring elements can be used to steer and focus the sound. For these gratings, the elements are defined such that only a small number of discreet signals (2 or 4 phases, for example) are needed to focus or steer the beam. The simplicity of the electronics and the defined regions on the transducer, however, means that these arrays are unable to dynamically focus and steer to the same extent as a fully controlled 2D array. Furthermore, these techniques still require circuits that provide phase shifts or multiple transmit signals. U.S. Pat. No. 4,307,613 describes a reconfigurable Fresnel zone plate with an electrode configuration that allows a focal point to be scanned linearly. However, because of the way the electrodes are defined, this phase plate is only scannable in one direction and not as fully flexible as a fully controlled 2D array. While electroding configurations may allow some flexibility in the focusing and steering of the array, ultimately they do not allow the full flexibility of steering of a full 2D array and also still require separate transmit signals in order to create a focus or steered beam.